1. Field of the Invention
The present invention relates to a novel process for the preparation of coatings on water-resistant substrates using an aqueous coating composition wherein the binder consists essentially of a hydrophilically modified, water-dispersible polyisocyanate having free isocyanate groups.
2. Description of the Prior Art
Compositions containing polyisocyanates having free NCO groups or isocyanate prepolymers have been known for a long time. They cure in the presence of moisture to form N-substituted polyurea compounds. Such products form the basis of one-component polyurethane coating technology (cf. e.g. Kunststoff Handbuch (Plastics Handbook), Volume 7, "Polyurethane" ("Polyurethanes"), Carl Hanser Verlag Munich/Vienna (1983), pages 545 et seq.).
Binders for the one-component compositions may be based on prepolymers prepared by reacting difunctional to polyfunctional isocyanate compounds with difunctional and/or polyfunctional alcohols. The NCO/OH equivalent ratio used for the preparation of the prepolymers is selected so that the prepolymers are soluble in organic solvents and also still contain free NCO groups. After application to a substrate, the NCO groups react with atmospheric moisture to provide crosslinked polyurea coatings. This produces films with a high crosslinking density and, thus, a high level of properties.
Conventional polyisocyanates derivatives, such as those generally used as crosslinking components in two-component polyurethane coating compositions, can be used for these applications, provided that they possess the property spectrum demanded by the market. These polyisocyanates derivatives may contain urethane, isocyanurate, uretdione, biuret and/or allophanate groups.
Because the NCO prepolymers and many of the polyisocyanate derivatives have viscosities of greater than 2000 mPa.s at room temperature, they require the addition of organic solvents to obtain a viscosity suitable for formulating and processing. Obviously, the higher the viscosity of the polyisocyanate derivative or NCO prepolymer, the greater the amount of solvent which must be added to obtain the necessary viscosity. For ecological and safety reasons (fire hazards), the use of coating compositions containing organic solvents is continually decreasing in practice. They are being increasingly replaced with aqueous coating compositions for coating a very wide variety of substrates.
The use of organic polyisocyanates having free isocyanate groups analogously to solvent-containing one-component systems as binders for aqueous lacquers has previously not seemed possible because these binders cure in the presence of moisture and there is an excessive supply of water in these aqueous compositions. Accordingly, it would not be expected to be possible to prepare aqueous compositions based on polyisocyanates having free isocyanate groups as essentially the only binder component.
The fact that aqueous two-component polyurethane coating compositions have been disclosed does not affect the preceding conclusion because, in the two-component systems, certain polyhydroxyl compounds are preferably used in combination with hydrophobic polyisocyanates. Therefore, it is possible to assume that the polyisocyanate is to some extent enveloped by the organic polyhydroxyl compound to prevent a reaction between the free isocyanate groups and water, which forms the continuous phase of the emulsions.
Surprisingly, however, it has now been found that certain hydrophilically-modified polyisocyanates or polyisocyanate mixtures, which are described in greater detail below, are excellent binders for aqueous coating compositions and provide a processing time which is sufficiently long for practical purposes.